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Natural background gamma radiation was measured along the main roads in the eastern, western and southern regions of Thailand using a car-borne survey system with a 3-in × 3-in NaI (Tl) scintillation spectrometer. The system was able to quickly survey a large area and obtain outdoor absorbed dose rate in air from a gamma ray pulse height distribution. A total of 19 219 data of the outdoor absorbed dose rate in air were collected. The average outdoor absorbed dose rate in air in the eastern, western and southern regions were found to be 8–127, 16–119 and 16–141 nGy h ^-1 , respectively. The highest outdoor absorbed dose rate in air was detected in the southern region of Thailand. The corresponding average outdoor annual effective dose to the public ranged from 11.7 to 80.8 μSv.